High molecular weight, water-soluble, linear polymers, for example polyacrylates and polymethyl acrylates, are known. These polymers are employed, for example, for increasing the viscosity of aqueous systems, as a suspending agent, for accelerating sedimentation and complexing, and in super-absorbers for binding water and in lacquers and varnishes which can be diluted in water. The fact that these products are not biologically degradable proves to be a disadvantage. Derivatized, highly polymerized polysaccharides come into consideration as an alternative. These polysaccharides have to date been obtained biotechnologically, by means of fermentation and transglycosylation. However, for economic reasons, fermentatively produced polymers are not suitable for relatively large-scale applications. For this reason, attempts have been made for some time now to produce linear, water-soluble polymers, such as inulin, in plants.
Inulin, which is a .beta.-2-1-linked polyfructan, can be detected as a storage carbohydrate in some dicotyledonous higher plants and is present at a molecular weight of 5-50 kD. In addition, among the bacteria, some Gram-positive and Gram-negative bacterial species are known to synthesize a related fructan polymer, i.e. the .beta.-2-6-linked levan, using so-called levan sucrases. Polyfructans which are formed in bacteria exhibit substantially higher molecular weights of up to 2000 kD. At present, only one Gram-positive bacterium, i.e. Streptococcus mutans, has been described which uses an ftf (fructosyltransferase) gene to form inulin on the basis of sucrose (Shiroza and Kuramitsu, J. Bacteriology (1988) 170, 810 to 816).
Processes which use biotechnological methods to alter the carbohydrate concentration and/or the composition of the carbohydrates in transgenic plants are known. PCT/US89/02729 describes one option for producing carbohydrate polymers, in particular dextran or polyfructose, in transgenic plants, in particular their fruit. The use of levan sucrase or dextran sucrase from a variety of microorganisms is proposed for producing these plants. The patent does not demonstrate the formation of the active enzymes nor that of levan or dextran, and nor does it demonstrate the preparation of transgenic plants.
PCT/EP93/02110 discloses a process for preparing poly-fructose-producing transgenic plants which contain the lsc gene for a levan sucrase from a Gram-negative bacterium.
PCT/NL93/00279 discloses the transformation of plants with chimeric genes which contain the sacB gene from Bacillus subtilis or the ftf gene from Streptococcus mutans. In the case of the sacB gene, a modification of the 5'-untranslated region of the bacterial gene is additionally recommended for increasing the level of expression in transformed plants. No sequence modifications for improving expression are described in the case of the fructosyltransferase gene from Streptococcus mutans. As a result, the level at which the fructosyltransferase is expressed is comparatively low.